This invention relates, in general, to a method of forming electrically isolated areas on a metallized substrate and is particularly directed to a method of making an electrostatic deflector for use in controlling an electron beam in an electron beam lithography machine.
In an electron beam lithography machine, there is a beam column with a centrally located electron beam which is controlled to perform a writing operation on a substrate, such as a wafer or mask, located below the beam column. In the beam column, the electron beam is within a central tube which has a section, called a minor field deflector, with electrostatic plates which deflect the beam +/-32 microns at a very high speed by electrostatics as part of the writing operation. In order to achieve this high speed, the electrostatic plates must have a very low capacitance and must be such that the electron beam is not adversely affected by exposed substrate. One such deflector is actually a glass tube approximately 6.3 inches long and about 0.6 inches ID and 0.75 inches OD. The tube is coated on the insides and the outsides with a thin film of gold approximately 10 to 20 microinches (0.25 to 0.50 microns) in thickness. The inside surface of this tube is divided into eight areas that are equally spaced, equal in area, and electrically isolated from one another to form electrostatic metal plates. In the beam column, each of these eight plates is electrically connected in pairs of opposite plates to a voltage source to create an electrostatic field to control the electron beam. Opposing plates have the same voltage value but of opposite polarity.
In order to achieve the high speed and provide very low capacitance, the deposited thin metal film on the inner surface of the substrate (glass) is divided into areas (plates) by scribing lines of a specific width and depth on the inner surface of the tube.
In a prior art method of forming these electrostatic plates, the preo-coated glass tube was mounted in a holder and a reciprocating boring bar with a diamond scribe tip was inserted into the inner bore of the tube and withdrawn along a major portion of the length of the tube, while in engagement with the coating, to scribe lines which separated the coating into separate areas. The scribe tip was held against the inner wall by the bias of a spring during the scribing step, and after each scribing step, the tube was rotated and the drawing step repeated to form the eight separate plates.
In practice, there were two ways in which the scribe tip was spring loaded. In one setup the scribe tip was mounted on a short arm pivotally connected to the boring bar. The pivotal arm was spring biased toward the tube. It was found that the pivot point was too loose and the scribe tip wobbled from side to side. In another setup the scribe tip was mounted on a leaf spring which was mounted on the boring bar. This method also failed. The main problem was that the lines were not cut all the way through the metal coating; thus, separate plates were not created. These setups also did not allow re-scribing since the scribe lines were not exactly on top of one another and thus would create two scribed lines which was unacceptable. Thus the two prior art methods were essentially a one time operation and could not be redone. This caused low yield and high scrap which increased manufacturing cost.
Other methods of creating isolated plates have been suggested, for example, etching the lines between the plates but the separation by these methods is too wide leaving large areas of the substrate which would cause a charging of the substrate by the electron beam which would in turn disturb the beam. It has also been suggested that the substrate be covered with a charge drain coating to correct the effect of these wide lines but this is an additional and costly step. The present invention avoids this extra coating by minimizing the area of exposure of the substrate to the electron beam.
It is thus an object of this invention to provide an improved method of creating isolated electrostatic plates on a field deflector so that the plates are electrically isolated from one another with a spacing that will not create a disturbance of the electron beam being controlled within the deflector.